The invention relates to a microsensor with a resonator structure, which is excited by first electrical signals to oscillate and emits second electrical signals in dependence on the measuring variable, wherein for the thermal excitation of oscillations a heating element is arranged on the resonator structure to which is supplied at least one of the first electrical signals.
An acceleration sensor with a resonator structure that is thermally excited to oscillate is known from xe2x80x9cAn Integrated Resonant Accelerometer Microsystem for Automotive Applications,xe2x80x9d Transducer ""97, pages 843-846. With this known acceleration sensor, a seismic mass is suspended on two arms in the center of a recess in the silicon chip. The two arms extend diagonal. A resistor is arranged on one of the two arms, which is supplied with a pulsed current. Owing to the thermal stress that changes over time as a result of the temperature differences between the top and the bottom, the structure is excited to oscillations that are adjusted to a resonance requirement by means of an outside wiring. A force exerted onto the seismic mass, for example resulting from an external acceleration perpendicular to the surface of the structure, causes a change in the resonance frequency, which is detected and evaluated.
The thermal excitation by means of current pulses through resistances that are diffused in the surface generates oscillations, for which the oscillation plane is at a right angle to the resonator structure, thereby resulting in a number of disadvantages. In particular, the achievable temperature difference between front and rear side of the heated web is very low, owing to the spatial nearness and the poor thermal insulation. Thus, a relatively high heating output is required. Furthermore, the sensitivity direction of this sensor is perpendicular to its surface. This is not desirable for a plurality of applications, in particular for the use as acceleration sensor for passenger protection systems in motor vehicles. However, for production-technological reasons, vertically oscillating resonators are not very suitable for use as locally sensitive sensors because a different thickness is required, for example, for the resonator and the seismic mass.
It is the object of the invention to specify a microsensor with a resonator structure, for which a lateral thermal excitation of the oscillations occurs.
This object generally is solved according to the present invention with a microsensor having a resonator structure that is excited by first electrical signals to oscillate and which emits second electrical signals in dependence on a sensed measuring variable, wherein for the thermal excitation of oscillations, a heating element is arranged on the resonator structure, which heating element is supplied with at least one of the first electrical signal, at one base connector point of at least one oscillating segment or portion of the resonator structure, the microsensor has at least two regions that are thermally separated by a zone with reduced heat conductance and the heating element is arranged on one of the two regions, and excites the resonator structure to lateral oscillations. Advantageous embodiments and modifying of the microsensors are disclosed and discussed.
In the case of a microsensor with a resonator structure and thermal excitation of the oscillations, at least two regions that are thermally separated by a zone with decreased heat conductance are provided at one base connection point of at least one oscillating segment of the resonator structure, with the heating element being arranged on one of the region. As a result of this arrangement, lateral oscillations can be excited in the resonator structure if the heating element is provided with corresponding current pulses. It is advantageous if a recording element is arranged at least one of the other regions to detect the oscillation amplitude.
The heating element for one advantageous embodiment of the invention is used simultaneously for the detection of the oscillation amplitude.
A first embodiment of the invention provides that the zone with reduced heat conductance comprises a mechanical recess, which produces an increased thermal insulation between the resulting two regions.
A second embodiment of the invention provides that the oscillating portion of the resonator structure consists of a resonator web, fixed at one or several locations, which changes to a U-profile at its base connections point. In this case, the heating element is arranged on the first leg of the U-profile and the receiving element on the second leg of the U-profile.
In order to sense the measuring variable, it is advantageous if the resonator web is fixed with the end facing away from the U-profile to a sensor-specific structure, which leads to a detuning of the resonance frequency. In the embodiment as acceleration sensor, this is a seismic mass.
Another embodiment of the invention provides that the oscillating part of the resonator structure consists of a tuning fork, having a recess arranged near the base point. This recess provides a thermal insulation between a web that connects the tins of the tuning fork and the base point of the turning fork. The heating element in that case is arranged advantageously on the web.
The resonator structure consists of a semiconductor material, preferably a mono-crystalline silicon. The heating element is designed as doped resistance zone in the silicon material or as thin-film resistor on the surface of the resonator structure.
The receiving element for detecting the oscillation amplitude is designed as piezoresistance. A capacitive detection of the oscillation amplitude is also possible.